In the further, process control is expected to be very important, when an electron microscope or an optical inspection device is used, with miniaturization of semiconductor devices. PTL 1 discloses a film thickness measuring device that measures a film thickness of a thin film, during a flattening process of the thin film as one process of semiconductor manufacturing processes. PTL 1 describes the film thickness measuring device that irradiates the thin film with white light so as to evaluate a state of processing performed using a chemical mechanical polishing (CMP) method, and performs film thickness measurement, on the basis of spectral analysis of reflected light.